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NASA’s Cassini spacecraft made its last fly-by of Enceladus on Dec. 19. Since then it has been transmitting to Earth images taken of the ocean moon, including this beauty received Feb. 15:

This view of Enceladus is from about 83,000 kilometers. The moon is captured in winter. It’s north pole is to the upper left, while its south pole is obscured by darkness in the lower right. Courtesy NASA, Jet Propulsion Laboratory, Space Science Institute, Justin Cowart.

The view was acquired at a distance of approximately 837,000 miles (1.35 million kilometers) from Enceladus, with an image scale of 5 miles (8 kilometers) per pixel. Tethys was approximately 1.2 million miles (1.9 million kilometers) away with an image scale of 7 miles (11 kilometers) per pixel. Mimas was approximately 1.1 million miles (1.7 million kilometers) away with an image scale of 6 miles (10 kilometers) per pixel.

Tethys’ diameter is about 1,066 kilometers. The cratered moon orbits Saturn from a distance of about 294,600 kilometers. That is about 20 percent farther from Saturn than the Moon is from Earth. Tethys is slightly less dense than liquid water, which suggests that its structure is mostly ice. This frigid moon is tidally locked to Saturn – it does not rotate and only one side of Tethys faces Saturn.

Enceladus is smaller than Tethys, with a diameter of about 500 kilometers. The host of a sub-surface liquid water ocean, Enceladus is covered by water ice and reflects nearly all the sunlight that hits it. That makes it a very cold satellite. The surface temperature on the moon is about -201 degrees Celsius. It orbits Saturn from a mean distance of about 238,000 kilometers, which is about the same distance as Earth’s satellite is from our planet.

This whimsical poster depicting Enceladus is part of NASA’s Visions of the Future project. You can see more at http://www.jpl.nasa.gov/visions-of-the-future/. Courtesy NASA, Jet Propulsion Laboratory – California Institute of Technology.

Mimas is the smallest of Saturn’s major moons. It is heavily cratered, with a giant crater called Herschel stretching across about one-third of its surface. That feature has led Mimas to sometimes be called the “Death Star moon” because it evokes the fictional planet-destroying spacecraft in the Star Wars films. Mimas’ diameter is about 400 kilometers; it orbits Saturn from a mean distance of about 200,000 kilometers.

Enceladus, the icy moon of Saturn known for its spectacular geysers, likely has an ocean of liquid water beneath its surface.

Researchers relied on gravity and topography data obtained by NASA’s Cassini probe in concluding that Enceladus could be the third known outer solar system moon that features liquid water.

The ocean is a regional water body near the moon’s south pole. It seems to lie about 30 to 40 kilometers below the moon’s surface, covered by a thick ice sheet, and likely has a depth of about 10 kilometers.

“This means it is as large as or larger than Lake Superior,” David J. Stevenson, a professor of planetary science at the California Institute of Technology and a co-author of the paper documenting the discovery, said.

Scientists depended on measurements of the gravity field of Enceladus to infer the presence of the ocean. As Cassini flew 100 or fewer kilometers over the moon’s surface during three flyovers, the spacecraft’s speed changed by a few millimeters per second.

The tiny slowing of Cassini was detected by antennae on Earth as the radio waves transmitted by the probe during the flyovers shifted slightly, just as sound waves do as their source moves farther away from a receiver.

“The detection of such small velocity variations gave us information about the accelerations the spacecraft was subject to,” Marzia Parisi, an aerospace engineer and research fellow at Sapienza University of Rome and one of the authors of the paper, said.

An acceleration of an object can include both an increase and a decrease in its speed.

The acceleration of a spacecraft that is flying over a celestial object is affected by a variety of factors, including changes in the object’s gravity.

“This could arise because of the absence of mass that is a depression of the ice surface, but when you look at the actual surface, you can see that although there is indeed a depression, it is much larger than that needed to explain the gravity,” Stevenson said. “So that means that there must be a compensating positive mass excess under the south polar ice. The natural way to do this is to have a layer of water because water is more dense than ice.”

In the case of the Cassini flyovers, another likely impact on the spacecraft’s velocity was drag caused by passing through water vapor plumes that are emitted from the moon’s south polar region.

The impact of those factors had to be considered before the research team could conclude that the cause of Cassini’s acceleration change was a fluctuation in mass tied to the presence of liquid water beneath the moon’s surface.

Parisi explained that the researchers were able to take account of those factors in their modeling.

The data does not explain how an ocean could exist on Enceladus, but it is logical to assume that Saturn’s gravitational pull has a great deal to do with it.

One significant implication of the discovery of an ocean on Enceladus is the possibility that it could be hospitable for microbial life.

“The added value of our work is that a large, potentially habitable environment has been found in an unexpected place in the solar system, where the energy needed to produce liquid water from ice is not provided by solar radiation,” Luciano Iess, a professor of aerospace engineering at Sapienza University of Rome and the lead author of the paper, said.

It is likely to be many years, if not decades, before humanity can find out whether, in fact, microbes exist beneath Enceladus’ icy surface.

Cassini will continue to explore the Saturnian system for about three more years. The probe will fly over Enceladus three more times, but will not further investigate Enceladus’ gravitational field to gain further insights into the ocean beneath its surface.

Europa, a Jovian moon, and Titan also have oceans, though on Titan it is composed of liquid methane.

NASA’s planetary science budget includes funds to plan a trip to Europa.

The paper documenting the discovery of an ocean on Enceladus will be published in the April 4, 2014 edition of Science.

Image courtesy Wikimedia.

Graphic courtesy NASA, Jet Propulsion Laboratory-California Institute of Technology. Note the illustration of the hypothesized sub-surface ocean, appearing as a blue region just above the water vapor jets.